I used an OS 160 twin modified for gas. It has ample power swinging an 18-6 prop at 7200 RPM. The ignition system is available from CH Ignition. They will also convert your engine for you if you wish. The fuel I use is 20 to 1. To eliminate the mess from the crankcase drain simply drill and tap a hole in the intake manifold and install a fuel nipple. Connect a piece of fuel tubing from the crankcase drain to the nipple. Away goes the mess. I have used this on all my 4 strokes for years since OS started to do it on their four stroke engines.
Fred